1. Field of the Invention
The present invention relates to tractors for feeding continuous webs as are generally used in printers and computers.
2. Description of the Prior Art
Such tractors may include a pair of side frames, a driving and an idler pulley rotatably supported between the side frames at spaced intervals, and an endless feed belt trained around the driving and idler pulleys, the feed belt having on the outer surface thereof a predetermined number of feed pins engageable with equispaced perforations formed in a continuous web. One of these tractors is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 59-164744.
On such web feed tractors, it is common to provide certain mechanisms which serve to prevent disengagement of the feed pins from the perforations in the feed belt. Thus, in some designs, the opposite edges of the feed belt may be abutted against the inner walls of the side frames so as to restrain lateral meandering of the feed belt; and a belt drive support element is provided on the inner wall of one of the side frames so as to support the inner surface of the upper reach of the feed belt and thereby prevent deflection of the feed belt.
In such a conventional web feed tractor, however, during driving of the feed belt, sliding friction is produced between the inner walls of the side frames and the opposite edges of the feed belt and between the linear surface of the belt drive support element of the side frame and the inner surface of the feed belt. Then, it has been necessary to employ a driving unit for the driving pulley which has a larger output by the amount equivalent to the sliding frictional force, and such a larger output of the driving unit disadvantageously causes so much increase of equipment and operation costs.